1. Technical Field
The present invention is directed to methods for compressing sonic data obtained as the result of well logging procedures.
2. Background Information
Turning now to FIG. 1, a schematic diagram of a logging operation is shown. Tool, or sonde, 10 for acquiring sonic data is located in borehole 11 penetrating earth formation 12. Optionally, the borehole wall may have casing 13 cemented thereto, e.g., in a production well. The sonde is preferably lowered in the borehole by armored multiconductor cable 14 and slowly raised by surface equipment 15 over sheave wheel 16 while sonic data measurements are recorded. The depth of the tool is measured by depth gauge 17, which measures cable displacement.
Sonde 10 acquires sonic data by emitting an acoustic pulse and recording its return waveform. The sonde comprises at least one sonic source, or transmitter, and at least one detector, or receiver. The source typically produces a pulse upon excitation. The pulse travels through the casing and formation, and back to the sonde where it is detected by the receiver(s) thereon. The return waveforms can be analyzed by the sonde in situ, analyzed by data processor 18 at the surface, or stored, either in the sonde or at the site, for analysis at a remote location. In the preferred embodiment, the return waveform data is transferred to data processor 18 by cable 14 for analysis at a remote location.
Sonic data acquired in this manner is typically displayed on a chart, or log, of waveform amplitude over time versus depth. With reference to FIG. 2, a representation of sonic data in log format is shown. The data was recorded at 200 successive depth locations, the depth interval consisting of various formations, ranging from very hard limestone to very soft shaley sand.
Ideally, the waveform data records the arrival of compressional (P) waves, shear (S) waves and Stoneley (ST) waves. However, the P, S and/or ST wave components often contain undesirable components which degrade and/or mask the desired wave components. Examples of undesirable components include reflected and/or converted waves, casing arrivals, tool arrivals and noise.
The P, S and/or ST wave components are often contaminated by reflected and/or converted waves when the receiver is close to a bed boundary. Reflected waves are waves which are emitted by the source as P, S or ST waves, reflected by bed boundaries, and received as P, S or ST waves, respectively. Converted waves, on the other hand, are waves which are emitted by the source as P, S or ST waves, reflected by bed boundaries, and received as something other than P, S or ST waves, respectively. For example, a P wave which is "converted" to an S wave, and so recorded, is a converted wave.
Several examples of reflected and converted waves are marked at A through I in FIG. 2. As can be seen in FIG. 2, the reflected and converted waves occur at the same time as the desired waveform arrivals. Additionally, the frequency content of the reflected and converted waves overlaps that of the P, S and/or ST wave components.
Aside from P, S and ST wave component arrivals from the formation, the acoustic pulse travels through the steel casing, as well as the tool itself. The waveforms therefrom are commonly referred to as casing and tool arrivals, respectively. The point in time at which a waveform component arrives and is subsequently detected by the sonde's receiver(s) is commonly referred to as the waveform component's arrival time.
A steel casing is typically cemented to the borehole wall in production wells. As is well known, the cemented casing isolates the various water, gas and oil bearing zones from each other, thereby maintaining zone integrity. When the cement bond is good, the casing is substantially transparent to the sonic tool. Thus, the sonic tool is capable of logging through the casing and acquiring the formation wave components. However, when the cement bond is deficient or deteriorated, a strong wave travels through the casing, subsequently detected by the receiver(s). The casing arrival can mask the formation arrivals which have a smaller amplitude.
The tool itself also provides a path from transmitter to receiver. The tool's arrival time can appear at the same time as the formation arrival, based on tool design. Further, the amplitude of the tool arrival can be several times that of the formation arrival. As a result, the formation wave is masked by the tool arrival. As appreciated by those skilled in the art, the formation wave is desired for many reasons, e.g., to calculate the formation slowness.
Additionally, the recorded waveforms are likely to be contaminated by noise, e.g., geological, external, electronic and/or quantization. Geological noise is generated by the formation, e.g., from fracture development. External noise is due to interference from external sources, e.g., road traffic, rig activity and the like. Electronic noise is produced by the electronic components of the sonde, e.g., due to component reaction to thermal fluctuations, cross-talk or shot-noise. Quantization noise is the result of waveform degradation inherent in digitizing an analog signal, such as the waveforms acquired by the sonic detectors. Since noise tends to degrade and mask the acquired data, noise is undesirable.
Numerous filtering techniques are known for removing undesired components from waveforms. For example, the waveform can be filtered in either the temporal or the spectral domain. Such techniques, however, cannot be applied to sonic data to remove undesired components caused by reflected and/or converted waves, casing arrivals, tool arrivals and/or noise. In order for temporal or spectral filtering to be effective, there must exist a time or frequency separation, respectively, between the desired and undesired components. Such separation does not exist between the desired and undesired components in the acquired waveform.
Additionally, the waveforms can be filtered using velocity filtering techniques. Reflected waveforms appear at a receiver after the direct waveform, typically having similar frequency characteristics, albeit different velocities (i.e., arrival times). In order to separate the two, a velocity filter is employed which separates arrivals based on their different velocities.
However, velocity filtering does not filter out noise components. Additionally, conventional filtering techniques do not allow the identification of lithology, let alone allow for data compression.